ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



71 



destroyed ; and the fissures have a radial direction crossing the layers 

 at right angles, a proof that more water is lost in the tangential than 

 in the radial direction, and that the total quantity of water deposited 

 in the tangential direction is greater. When substances which 

 cause artificial swelling act slowly on the naturally saturated starch- 

 grain, it increases in volume, radial fissures being again formed, a 

 proof that during this process more water is deposited in the radial 

 than in the tangential direction. He argues, on mechanical grounds, 

 that the tensions found in starch-grains can be accounted for only by 

 intussusception, and that these tensions can cause the secretion of 

 the soft nucleus and the soft layers only on the supposition that 

 intussusception is at the same time taking place. 



CoUenchyma.* — H. Ambronn has carefully investigated the his- 

 tory of develoj)ment and the mechanical properties of collenchyma 

 in a number of instances, especially in Colocasia esculenta and other 

 allied aroids, and in Umbel] if erae and Piperacese. 



With regard to the history of its development, these observations 

 confirm the statement of Haberlandt that, as in the case of bast, no 

 uniform origin can be ascribed to the collenchyma, but that it varies 

 in every possible way. Also that the grouping and arrangement of 

 the cells is the result, in the first place, of purely mechanical and not 

 of morphological laws ; and that, when definite relationships exist 

 between the collenchyma and the mestome (in Schwendener's sense of 

 the term), these relationshijis are explained by the history of develop- 

 ment. These relationships occur in those plants in which the origin 

 of the collenchyma and of the mestome is uniform, and in those in 

 which projecting ridges or angles are produced by the formation of 

 vascular bundles at the periphery, groups of coUenchymatous cells 

 being developed in them in consequence of their centrifugal tendency. 



As regards the structure of the coUenchymatous cells, they have 

 in general a prosenchymatous character. They are moderately long, 

 often 2 mm. or more, and very frequently manifest subsequent seg- 

 mentation by delicate division-walls. They are always filled with 

 sap, but contain little or no chlorophyll. The longitudinal walls of 

 the cells have usually longitudinal crevice-like pores. 



Other coUenchymatous cells, on the contrary, have more of a 

 parenchymatous character, and have usually been formed by secondary 

 coUenchymatous thickening of parenchymatous cells. 



The cell-walls of collenchyma are always coloured a bright blue 

 by chlor-iodide of zinc, but are not coloured by the action of phloro- 

 glucin and hydrochloric acid. Their power of swelling in water is 

 not so strong as has usually been supposed ; the cells are seldom 

 contracted by more than ^ per cent, of their entire length by the 

 application of desiccating reagents. 



The elements of the formative tissue out of which the coUenchy- 

 matous cells are subsequently developed are partly cambial, partly 

 belonging to other meristematic portions. But very often there is 

 no special formative tissue ; the coUenchymatous thickening taking 



* Priugslieim's Jahrb. wiss. Bot., xii. (1881) pp. -±73-511 (6 pis.). 



